Gas turbines for e.g. power generation are normally protected by air filters provided in the air intake system. The purpose of these filters is to protect the engine from residue build-up, fouling, on the turbine blades in the compressor stage. This type of accumulation of unwanted material reduces efficiency and increases probability of failure of the gas turbine. Insufficient filter performance can also cause engine corrosion in the decompression stage (hot end corrosion). In the past, it was common to use filters in filter classes F7-F9 (EN779-2002) having a removal efficiency of almost 100% for particles above 3 μm but merely 50-75% for particles sized 0.4 μm and below. It has, however, been revealed that particles having a size below 0.4 μm have a significant impact on both fouling and hot end corrosion and this has lead to an increased demand for filters in higher filter classes. Today, the use of filters of filter class EPA11, removal efficiency of 95% on Most Penetrating Particle Size (MPPS) or even EPA12, removal efficiency of 99.5% on MPPS, is not unusual. A problem with these highly efficient filters is that they have a considerably higher pressure drop. In new installations, this could be compensated for by using a larger filter area to reduce the air flow per area unit. In existing gas turbines, this is not always possible, or at least not feasible for cost reasons and the use of more efficient filters is therefore prevented. New materials allow the combination of high filter efficiency and low pressure drop. Examples of materials are porous membranes of expanded polytetrafluoethylene (ePTFE), ultra high molecular weight polyethylene (UPE) or membranes of electro spun polymer fibers. These types of membranes have in common that they are very thin and have a low fiber density with fine fibers, thus generating large fiber surface area which is a key parameter in achieving high filtration efficiency while not restricting the air flow. These thin membranes are, however, sensitive to damage. Another problem is that the passage of air will be restricted due to agglomeration of particles on the upstream surface of the membrane. The membrane typically has an active filtration thickness between 0.01 mm and 0.1 mm while standard filters made of cellulose or synthetic fibers have a thickness of 0.3-1 mm. The greater depth of the standard filters leads to particle capture within a greater volume such that flow restriction due to particle agglomeration takes longer time to build up. This can be solved by laminating a pre-filter layer on an upstream surface of the membrane that captures a majority of the larger particles (>3 μm) thus reducing the particle agglomeration on the membrane. EP-1674144B1 discloses a system comprising a gas turbine and a filter having a filter media with a porous polymeric membrane and a depth filtration layer comprising electrostatically charged polymeric fibers. The electrostatic charging of the depth filtration layer is done to enhance filter performance. This solution has a number of disadvantages, the most noticeable being the fact that the electrostatic charge of the depth filtration layer is only temporary so that filter efficiency will diminish with over time.